1. Field of Invention
This invention relates to regenerators including regenerators for regenerative gas cycle machinery, and in particular, to tab connectors linking separate pieces of etched regenerator foil.
2. Description of Prior Art
Regenerative gas cycle machines are a class of machinery that includes Stirling cycle engines and Stirling cycle, Gifford-McMahon and pulse tube refrigerators. A regenerator is a critical component of all regenerative gas-cycle machines. In theory, a parallel-plate configuration offers a more favorable relationship between heat transfer and pressure drop than any other regenerator configuration, maximizing regenerator effectiveness. To make a parallel plate regenerator with the tight flow passages required for service in regenerative gas cycle machinery, spaced layers of foil have been tried. In practice, performance of foil regenerators often has been disappointing. In part, that disappointing performance has been due to difficulty in creating and assembling foil regenerators with uniform flow channels.
Regenerative gas cycle machines, including both engines and refrigerators, have been constructed with annular regenerators. Those regenerators have been constructed with a continuous spiral wrap of solid metal foil using ridges or dimples in the metal to separate the layers from each other. However, because it is difficult to create dimples of uniform depth and because there can be no cross-flow through the solid foil to adjust pressure differences between different layers, uniform flow patterns have not been achieved and performance of foil regenerators has been limited.
Some of the problems of foil regenerators are met by using a photo-etched sculpted foil regenerator disclosed in U.S. Pat. No. 5,429,177, which allows cross-flows through perforations in the layers of foil. My co-pending application Ser. No. 09/903,302 describes improved patterns for etched foil materials that further improve regenerator performance. However, it is difficult to make regenerator foil in lengths exceeding about 1 meter by batch processes of photo-etching and prohibitively expensive to make it in small quantities in continuous form. For best performance, all regenerator foil should be of the same density. However, it is difficult to make large pieces of photo-etched regenerator foil of uniform density. Thus, to maximize the yield of usable material emerging from the etching process, it is desirable to manufacture etched regenerator foil in strips substantially less than one meter long. Except for the smallest cryocoolers, a single piece of etched regenerator foil is too short to make a complete regenerator and several pieces of etched foil must therefore be spliced to make a spiral-wrapped regenerator.
Splicing foils end-to-end is difficult because the foils are thin and delicate, thus difficult to align with the required precision, and subject to damage in handling. Welding and gluing are two methods of splicing foil that have been tried. Both are difficult and expensive. Stringent requirements with respect to outgassing limit the bonding materials that can be used to join the ends of a foil strip to be used in a cryocooler application. No fully satisfactory, inexpensive method of splicing has been demonstrated heretofore.
Moreover, if a long strip of regenerator foil is welded or glued together from several shorter strips of etched regenerator foil material, the long strip becomes difficult to handle without damaging the foil. For satisfactory performance as a regenerator, the foil must be rolled tightly. Any kinks or ripples in the foil will tend to prevent the adjacent layers of foil from lying tightly against the kinked or rippled portion. Thus, a strip of welded regenerator foil requires especially careful handling, adding to the expense of assembly.
The use of tabs to connect separate pieces of etched foil is known to the etched foil art in applications such as coffee machine filters. However, in prior art tab arrangements, a tab passes through a hole in another piece of foil (or another part of the same piece of foil) and locks into place leaving a portion of the hole open. That arrangement is unsatisfactory in a foil regenerator because the open hole would create a flow path for fluid the full thickness of the foil and thus larger than flow paths etched into the surface of the foil. It would permit fluid moving in flow channels on adjacent layers of foil to short-circuit through the open hole instead of passing through the intended passages etched into those layers. The large flow path created by the opening would thus spoil the even distribution of flow throughout the regenerator, reducing its effectiveness. Prior art tab arrangements in which the tab retains the full thickness of the foil from which it etched are also unsatisfactory because the joint would be thicker than a single layer of foil, and would creates a linear lump in the regenerator that would open up large, unwanted flow passages adjacent to the joint.
The tab connection of this invention solves problems in assembly of etched regenerator foil into finished regenerators. It permits connection of consecutive pieces of regenerator foil with joints that are no thicker at any point than the parent foil from which the regenerator foil has been etched. It seals the holes in the foil through which tabs are inserted, eliminating leak paths that would otherwise spoil the performance of a regenerator assembled using prior art tab arrangements. A half-etched tongue on a tab can prevent distortion of the tab when the joint is placed in tension during the process of rolling the regenerator.
Several objects and advantages of this invention are:
(1) To provide a high performance foil regenerator for use in gas cycle machines.
(2) To provide reliable joints between adjacent pieces of regenerator foil.
(3) To provide joints between adjacent pieces of regenerator foil that are no thicker than the parent foil from which those adjacent pieces of regenerator foil have been fabricated.
(4) To provide joints that do not create unintended leak paths through etched foil regenerators.
(5) To improve yield and quality of etched foil elements to be employed in etched foil regenerators.
(6) To provide high performance foil regenerators for use in regenerative gas cycle machinery.